Magnetic Materials Having Superparamagnetic Particles

ABSTRACT

Magnetic materials and uses thereof are provided. In one aspect, a magnetic film is provided. The magnetic film comprises superparamagnetic particles on at least one surface thereof. The magnetic film may be patterned and may comprise a ferromagnetic material. The superparamagnetic particles may be coated with a non-magnetic polymer and/or embedded in a non-magnetic host material. The magnetic film may have increased damping and/or decreased coercivity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/458,112, filed on Jun. 10, 2003, incorporated by reference herein

FIELD OF THE INVENTION

The present invention relates to magnetic materials and, moreparticularly, to magnetic materials having superparamagnetic particlesassociated therewith.

BACKGROUND OF THE INVENTION

A general trend in both storage and electronics has been to make devicessmaller and faster. The same trend exists with magnetoelectronic“spintronic” devices, wherein the magnetic regions are becoming eversmaller. Such devices rely, in some cases, on the switching of a regionof magnetic material between two or more stable configurations ofmagnetization, and in other cases (e.g., magnetoresistive fieldsensors), on biasing of the magnetization away from a single equilibriumconfiguration by, and in some proportion to, the field being sensed. Asmagnetic devices are made smaller, and designed to switch faster, thechallenge of getting the switched region to relax with certainty to adesired magnetization configuration arises. However, the conductingferromagnetic materials used in these devices, typically transitionmetal alloys, have very small intrinsic magnetization damping.

While a switching of the magnetization configuration of a device, e.g.,by an externally applied magnetic field or by spin injection, ideallychanges the magnetic configuration without any unwanted motion of themagnetization, in practice this rarely occurs. The magnetic region ofthe device typically processes rapidly with a large but graduallydecreasing amplitude following switching. During the precession period,particularly during the initial stages where the amplitude is large,spin-waves, both thermally excited and those excited by the switchingprocess and defects in the material, can interfere constructively todirect the magnetization to an equilibrium configuration other than theone intended. Even a small probability of such a spurious switchingevent (i.e., less than 1%) is unacceptable for memory applications. Inthe case of field sensors, it is also desirable to reach equilibriumquickly, i.e., to reduce the oscillatory response caused by precessionof the magnetization about the new equilibrium defined by the fieldbeing sensed.

A magnetic switching device or magnetoresistive sensor may comprise amagnetic tunnel junction (MTJ) device or a spin valve (SV) device. AnMTJ, in a simplest form, may comprise a stack of two ferromagneticlayers separated by a tunnel barrier at a cross-point of two conductors,one of which may be a word line (WL) and the other a bit line (BL). AnSV can be made by replacing the tunnel barrier in an MTJ with aconducting layer. In both cases, the resistance of the device dependsstrongly on the orientation of the magnetization of the two magneticlayers relative to each other. One of the two magnetic layers is oftenreferred to as a “free” magnetic layer or as a “storage layer.” Thestorage layer may comprise a single ferromagnetic or ferrimagneticlayer, or a synthetic antiferromagnetic structure with more than oneferromagnetic layer separated by one or more non-magnetic spacer layers.The magnetic orientation of the storage layer can be changed by thesuperposition of magnetic fields generated by programming currentsI_(WL) and I_(BL) flowing in the conductors WL and BL, respectively. Theother of the two magnetic layers is often referred to as a “fixed,”“pinned” or “reference” magnetic layer. The magnetization of the fixedlayer is invariant, and as such, the programming currents I_(WL) andI_(BL) do not change the magnetic orientation of this fixed layer. Thefixed layer can also comprise a single ferromagnetic or ferrimagneticlayer or a synthetic antiferromagnet consisting of more than oneferromagnetic layer separated by one or more non-magnetic spacer layers.The logical state (e.g., a “0” bit or a “1” bit) is generally stored inthe MTJ by changing the orientation of the free magnetic layer relativeto the fixed magnetic layer. When both magnetic layers have the sameorientation, the MTJ typically has a low resistance associatedtherewith, as measured between conductors WL and BL. Likewise, theresistance of the MTJ is generally high when the magnetic layers areoriented in opposite directions with respect to one another.

Another way of operating an MTJ is by active readout, wherein the MTJconsists of a storage layer and an “interrogation” layer. The storagelayer stores the “state” of the memory. The state of the memory may bedetermined by reading the output of the device as per the two possibleorientations of the interrogation layer. Both the storage and theinterrogation layer may be single ferromagnetic or ferrimagnetic layersor synthetic antiferromagnetic structures, as described above.

A conventional magnetic random access memory (MRAM) generally includes aplurality of MTJ devices connected in an array configuration. ExemplaryMRAM arrays include cross-point arrays, wherein each memory cellcomprises a single MTJ device connected at an intersection of a wordline and a corresponding bit line, and an architecture employing aplurality of memory cells, each memory cell comprising a selectiontransistor coupled in series with an MTJ device. The selectiontransistor is used for accessing the corresponding MTJ device during aread operation. MRAM circuits are discussed in further detail, forexample, in W. Reohr et al., “Memories of Tomorrow,” IEEE Circuits andDevices Mag., v. 18, no. 5, p. 17-27 (September 2002), the disclosure ofwhich is incorporated by reference herein.

The magnetization dynamics of magnetic materials are affected by theproperties of the magnetic materials. Magnetic material properties ofparticular importance for magnetic switching devices andmagnetoresistive sensors include damping and coercivity. Specifically,damping is the action whereby the amplitude of magnetic precession(oscillatory response) is decreased. Coercivity is a property of amagnetic material wherein the magnetic field required to return themagnetization of a magnetic material from saturation back to zero ismeasured. The coercive field, H_(c), can be used to approximate themagnetic field needed to switch the magnetization, i.e., the switchingfield.

One challenge associated with magnetic switching devices, such as thoseemploying small area structures, is that the shape anisotropycontribution to the switching field, assuming that the small magneticelement has an aspect ratio not equal to one in the plane of the device,increases inversely proportional to the thickness of the small magneticelement (perpendicular to the plane). This effect calls for the use ofhigher currents to generate the switching fields and brings aboutincreased power consumption and increased heating. It is thereforedesirable to be able to contain or decrease the coercivity of smallmagnetic elements.

The materials used in the above devices are typically ferromagnetictransition metal alloys. These materials tend to be severelyunderdamped, whether in a bulk form, a thin film or a smalllithographically defined element. Certain alloys have been shown to havedesirable enhanced damping properties in bulk (approximated by thickfilm) form. For a detailed description of these alloys, see Ingvarssonet al., U.S. Pat. No. 6,452,240 “Increased Damping of Magnetization inMagnetic Materials,” (hereinafter “Ingvarsson”) the disclosure of whichis incorporated by reference herein. Ingvarsson demonstrates thatcertain material choices provide improved damping properties ofmagnetoresistive devices. It would be further desirable to benefit theswitching characteristics of a magnetic switching device by changing thestructure of these devices, without affecting the composition of themagnetic materials.

Examples of underdamped magnetic materials include Permalloy™ magneticfilms, a trademark of B&D Industrial & Mining Services, Inc., having thecomposition Ni₈₁Fe₁₉, which have been shown to exhibit magnetizationoscillations after magnetic switching. For a detailed description of themagnetization dynamics of these Permalloy™ magnetic films, see forexample, T. J. Silva et al., “Inductive Measurement of UltrafastMagnetization Dynamics in Thin-Film Permalloy,” J. Appl. Phys., v. 85,no. 11, p. 7849 (1999), and S. Ingvarsson et al., “Role of electronscattering in the magnetization relaxation of thin Ni₈₁Fe₁₉ films,” thedisclosures of which are incorporated by reference herein.

The damping properties of films comprising pure ferromagnetic transitionelements, such as nickel, iron or cobalt are known. The dampingproperties of these elements are characterized by damping parametersthat are too small to achieve optimal switching behavior in devices. Fora detailed description of the damping properties in such materials, seeJ. M. Rudd et al., “Anisotropic Ferromagnetic Resonance Linewidth inNickel at Low Temperatures,” J. Appl. Phys., v. 57, no., 1 p. 3693(1985); B. Heinrich et al., “Ferromagnetic-Resonance Study of Ultrathinbce Fe(100) Films Grown Epitaxially on fcc Ag(100) Substrates,” Phys.Rev. Lett., v. 59, no. 15, p. 1756 (1987); and Schreiber et al.,“Gilbert Damping and g-Factor in Fe_(x)Co_(1-x) Alloy Films,” Sol. St.Comm. v. 93, no. 12, p. 965 (1995) (hereinafter “Schreiber”), thedisclosures of which are incorporated by reference herein. It has alsobeen shown that alloys of these particular metals have dampingparameters in the same order of magnitude as the constituent metals, seeSchreiber; C. E. Patton et al., “Frequency Dependence of the Paralleland Perpendicular Ferromagnetic Resonance Linewidth in Permalloy Films,2-36 GHz,” J Appl. Phys. v. 46, no. 11, p. 5002 (1975), the disclosuresof which are incorporated by reference herein.

A magnetic switching device has been created with a switching thresholdthat is more stable than conventional switching devices. See Sun, U.S.Pat. No. 6,256,223, “Current-Induced Magnetic Switching Device andMemory Including the Same.” The magnetic switching device comprises twoelectrodes, at least one of which is ferromagnetic, and a singlenanoparticle in between the two electrodes. The electrodes with thenanoparticle therebetween form an electrical switch. Switching of thedevice centers on the magnetic moment of the particle. Namely, a currentis injected through the electrodes, and the nanoparticle therebetween,to overcome the magnetic moment of the particle and switch the device.Further, the magnetic switching device requires that a large diameternanoparticle, on the order of several hundred angstroms, be employed.

Accordingly, it would be desirable to provide a magnetic material withbeneficial properties for use in applications, such as magnetoelectronicdevices, including but not limited to magnetic switching devices andmagnetoresistive sensors. Beneficial properties include favorablemagnetization properties, namely, increased damping and decreasedcoercivity. Favorable magnetization dynamics (i.e., increased damping)help the devices reach an equilibrium magnetic state, following aperturbation, in a more predictable and accurate manner. Lowering ofcoercivity allows for a more energy efficient switching and reducesunwanted heating. By being able to employ a manetic material with thesebeneficial properties, more accurate and consistent devices may beproduced.

SUMMARY OF THE INVENTION

The present invention provides magnetic materials and uses thereof. Inone aspect of the invention, a magnetic film is provided. The magneticfilm comprises superparamagnetic particles on at least one surfacethereof. The magnetic film may be patterned and may comprise aferromagnetic material. The superparamagnetic particles may be coatedwith a non-magnetic polymer and/or embedded in a non-magnetic hostmaterial. The magnetic film may have increased damping and/or decreasedcoercivity.

In another aspect of the invention, a magnetic switching devicecomprising two magnetic layers with a barrier layer therebetween,wherein at least one of the magnetic layers comprises a magnetic filmcomprising superparamagnetic particles on at least one surface thereofis provided. The superparamagnetic particles may be spatially separatedfrom the magnetic film.

In yet another aspect of the invention, a magnetic random access memory(MRAM) is provided. The MRAM comprises a plurality of memory cells; anda plurality of word lines and a plurality of bit lines operativelycoupled to the memory cells for selectively accessing the memory cells,wherein at least one of the memory cells comprises a magnetic switchingdevice including a magnetic film comprising superparamagnetic particleson at least one surface thereof.

In a further aspect of the invention, a magnetic film comprising atleast one superparamagnetic particle on at least one surface thereof isprovided. The at least one superparamagnetic particle has a diameter ofabout three nanometers to about 12 nanometers. In a further aspect ofthe invention, a magnetic film comprising at least one superparamagneticparticle embedded therein is provided.

In yet a further aspect of the invention, a method for forming amagnetic film comprising superparamagnetic particles is provided. Themethod comprises the following steps. The magnetic film is formed. Thesuperparamagnetic particles are deposited on at least one surface of themagnetic film. The superparamagnetic particles may be deposited usingsputtering, evaporative, laser ablative, and/or self-assemblytechniques. The method may further comprise the following steps. Atleast one non-magnetic layer is deposited. The superparamagneticparticles embedded in the at least one non-magnetic layer are formed bythermal treatment.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary configuration of amagnetic film according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary configuration of amagnetic film according to another embodiment of the present invention;

FIG. 3 is a diagram illustrating an exemplary configuration of amagnetic film having superparamagnetic particles embedded in anon-magnetic host material according to an embodiment of the presentinvention;

FIG. 4 is a diagram illustrating an exemplary configuration of amagnetic film having multiple superparamagnetic layers according to anembodiment of the present invention;

FIG. 5 is a diagram illustrating an exemplary configuration of amagnetic film having superparamagnetic particles embedded thereinaccording to an embodiment of the present invention;

FIG. 6 is a diagram illustrating an exemplary self-assembly depositiontechnique according to an embodiment of the present invention;

FIG. 7 is a diagram illustrating a cross-section of an exemplarymagnetic element test structure suitable for ferromagnetic resonancemeasurements (FMR) according to an embodiment of the present invention;

FIG. 8 is a diagram illustrating a cross-section of an exemplarymagnetic switching device according to an embodiment of the presentinvention;

FIG. 9 is a plot illustrating magnetic susceptibility as a function offrequency in samples of magnetic films according to an embodiment of thepresent invention;

FIG. 10 is a plot illustrating damping coefficients as a function of anapplied direct current (static) magnetic field of the magnetic filmsamples according to an embodiment of the present invention; and

FIG. 11 is an atomic force microscopic image of a section of anellipsoidal magnetic elements array according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described below in the context ofillustrative magnetic materials and uses thereof. However, it is to beunderstood that the teachings of the present invention are not to belimited to any particular conformation or implementation thereof.

FIG. 1 and FIG. 2 are diagrams illustrating exemplary configurations ofa magnetic film. Specifically, FIG. 1 is a diagram illustrating anexemplary configuration of at least a portion of a magnetic filmcomprising superparamagnetic particles on at least one surface thereof.In the exemplary embodiment shown in FIG. 1, magnetic film 100 comprisessuperparamagnetic particles 102 on multiple surfaces thereof.

According to the teachings of the present invention, magnetic film 100may comprise a ferromagnetic material. Magnetic film 100 may be formedhaving a homogeneous or heterogeneous composition comprising anysuitable ferromagnetic materials including ferromagnetic metals, suchas, nickel, iron, cobalt and alloys thereof that may also includechromium, molybdenum, copper, boron or carbon. In an exemplaryembodiment of the present invention, magnetic film 100 is formedcomprising a nickel-iron alloy of the formula Ni₈₀Fe₂₀, for example,Permalloy™. Further, any of the above metals may be present, eitherindependently or in combination, in distinct layers or regions withinmagnetic film 100.

Depending on the particular application, the thickness of magnetic film100 may vary. According to one exemplary embodiment, magnetic film 100has a thickness of greater than or equal to about five nanometers, suchas in the range of between about five nanometers to about 200nanometers. According to another exemplary embodiment, magnetic film 100has a thickness of less than or equal to about five nanometers, such asin the range of between about 0.5 nanometers to about five nanometers.The film thicknesses provided herein are merely exemplary, and it is tobe understood that magnetic film 100 may be of any thickness, as long asthat thickness is sufficient to form a continuous film.

Magnetic film 100 further comprises superparamagnetic particle 102. Asuperparamagnetic particle is a particle that has long rangeferromagnetic order, but a volume that is small enough to have a thermalenergy comparable with its anisotropy energy, therefore making theequilibrium magnetization unstable. Superparamagnetic particle 102 maycomprise a nanoparticle. The classification of a particle as ananoparticle depends on size, measured as the diameter of the particle.The parameters for the nanoparticle of the present invention aredescribed below. As is also described below, the nanoparticle maycomprise a metal, i.e., a transition metal.

According to the teachings of the present invention, any suitabletechnique for forming nanoparticles may be employed. For example, amethod for nanoparticle formation is provided in Murray, et al., U.S.Pat. No. 6,262,129, “Method For Producing Nanoparticles of TransitionMetals,” the disclosure of which is incorporated by reference herein.The teachings of the present invention should, however, not be limitedto any of the particular teachings provided therein.

As highlighted above, the nanoparticle may comprise a metal, i.e., atransition metal. Suitable transition metals include, but are notlimited to, cobalt, nickel, iron, platinum, and alloys andintermetallics of at least one of the foregoing transition metals.Further, the procedures used to form nanoparticles may result in thenanoparticles having a polymer coating. The polymer coating may benon-magnetic, and as such, may act as a magnetic exchange decouplingbarrier around the nanoparticle.

The dimensions of the nanoparticle impact the particle characteristics.Particularly, the dimensions of the nanoparticle impact the magneticproperties of the nanoparticle. In accordance with the teachings of thepresent invention, the nanoparticle has a diameter in the range ofbetween about three nanometers to about 12 nanometers.

Superparamagnetic particle 102 may be in direct contact with magneticfilm 100 (FIG. 1), embedded in a non-magnetic host material and/orspatially separated from magnetic film 100, as will be described inconjunction with the description of FIG. 3. To be in direct contact withmagnetic film 100, superparamagnetic particle 102 may be on at least oneof the surfaces of magnetic film 100 (FIG. 1 and FIG. 2), withinmagnetic film 100 (FIG. 5), or both (not shown). As is shown in FIG. 1,superparamagnetic particles 102 are present on the surfaces of magneticfilm 100. Magnetic film 100 may comprise at least one superparamagneticparticle 102 on at least one surface thereof. Thus, magnetic film 100may comprise a plurality of superparamagnetic particles 102, oralternatively, a single superparamagnetic particle 102. FIG. 2 is adiagram illustrating an exemplary configuration of a magnetic film.Specifically, FIG. 2 is a diagram illustrating at least a portion ofmagnetic film 100 comprising a single superparamagnetic particle 102 ona surface thereof.

Superparamagnetic particle 102 may be attached to the surfaces ofmagnetic film 100 using any suitable adhesive techniques. In addition,cohesive forces, i.e., molecular attraction forces, are sufficient tocohere and retain superparamagnetic particle 102 on the surface ofmagnetic film 100. Thus, in an exemplary embodiment, superparamagneticparticle 102 is cohered to magnetic film 100 using cohesive forces.

Superparamagnetic particles 102 on the surfaces of magnetic film 100 maybe embedded in a non-magnetic host material. FIG. 3 is a diagramillustrating an exemplary configuration of at least a portion of amagnetic film having superparamagnetic particles embedded in anon-magnetic host material. Specifically, FIG. 3 is a diagramillustrating superparamagnetic particles 102 embedded in non-magnetichost material 302, forming a monolayer on magnetic film 100, i.e.,superparamagnetic layer 306. As shown in FIG. 3, and as may beapplicable to other embodiments herein, for example those illustrated inFIGS. 1-2, 4 and 6-8, superparamagnetic particles 102 embedded innon-magnetic host material 302 may be spatially separated from magneticfilm 100 by spacer layer 304. Spacer layer 304 prevents short rangemagnetic exchange interactions, while optionally permitting long range,weak interactions. Spacer layer 304 may comprise any material suitablefor spatially separating superparamagnetic particles 102 embedded innon-magnetic host material 302 and preventing short range magneticexchange interactions. Suitable materials for use in spacer layer 304include, but are not limited to, metals, insulator materials,semiconductor materials, and combinations comprising at least one of theforegoing materials.

Spacer layer 304 should have a thickness sufficient to prevent shortrange magnetic interactions. In an exemplary embodiment, spacer layer304 has a thickness of about ten angstroms to about 100 angstroms. WhileFIG. 3 includes spacer layer 304, this particular configuration ismerely exemplary and it is to be understood that superparamagneticparticles 102 embedded in non-magnetic host material 302 may be incontact with at least one of the surfaces of magnetic film 100.

As shown in FIG. 3, non-magnetic host material 302 may have a pluralityof superparamagnetic particles 102 embedded therein. However,non-magnetic host material 302 might have a single superparamagneticparticle 102 embedded therein (not shown). Suitable non-magnetic hostmaterials include, but are not limited to, non-magnetic metals,insulator materials, semiconductor materials, and combinationscomprising at least one of the foregoing non-magnetic host materials.

A plurality of superparamagnetic particles 102 may form multiple layerson at least one of the surfaces of magnetic film 100, as is shown inFIG. 4. FIG. 4 is a diagram illustrating an exemplary configuration ofat least a portion of magnetic film 100 having multiplesuperparamagnetic layers. As described above, cohesive forces aresufficient to cohere or attach superparamagnetic particles 102 tomagnetic film 100. These forces are also sufficient to cohere themultiple superparamagnetic layers to magnetic film 100. Further, as isshown in FIG. 4, the multiple superparamagnetic layers may be embeddedin a non-magnetic host material, e.g., non-magnetic host material 302(FIG. 3).

Further, in accordance with the teachings of/be present invention, themultiple superparamagnetic layers may have discontinuities throughout,yet still provide the desired properties in magnetic film 100. Forexample, the multiple superparamagnetic layers may contain a cluster ofsuperparamagnetic particles forming a superparamagnetic island, i.e.,superparamagnetic island 402. Depending on the desired application, aplurality of superparamagnetic islands, such as superparamagnetic island402, may be distributed over the entirety, or portions, of one or moresurfaces of magnetic film 100.

In an alternative embodiment of the present invention, superparamagneticparticles 102 may be embedded within magnetic film 100, as is shown inFIG. 5. FIG. 5 is a diagram illustrating an exemplary configuration ofat least a portion of a magnetic film having superparamagnetic particlesembedded therein. While FIG. 5 shows a plurality of superparamagneticparticles 102 embedded in magnetic film 100, this configuration ismerely exemplary and the teachings herein further encompass embodimentswherein a single superparamagnetic particle 102 is embedded in magneticfilm 100. Superparamagnetic particle 102 may be embedded in magneticfilm 100, for example, during deposition of magnetic film 100 byconventional electrochemistry techniques.

FIG. 6 is a diagram illustrating an exemplary self-assembly depositiontechnique that may be employed to deposit superparamagnetic particles102 on the surface of magnetic film 100. As is shown in FIG. 6, simpleevaporative deposition techniques may be employed. In step 600 of FIG.6, a plurality of superparamagnetic particles 102 are dispersed within asolvent 610, in vessel 608. Solvent 610 may comprise any solvent, orcombination of solvents, useful for evaporative deposition. Suitablesolvents include, but are not limited to octane, hexane or methane. Asis further shown in step 600 of FIG. 6, and in subsequent stepsthereafter, superparamagnetic particles 102 may be coated with apolymer. The polymer may be non-magnetic. As was described above, thenon-magnetic polymer coating may act as a magnetic exchange decouplingbarrier around the superparamagnetic particle.

The plurality of superparamagnetic particles 102 dispersed withinsolvent 610 may then be deposited on magnetic film 100. As is shown instep 602, a plurality of superparamagnetic particles 102 dispersedwithin solvent 610 are spread over a surface of magnetic film 100. Forsimplicity, the depiction in FIG. 6 shows a plurality ofsuperparamagnetic particles 102 dispersed within solvent 610 beingspread as a single layer over only a single surface of magnetic film100. However, it is to be understood that according to the techniquesdescribed herein, the plurality of superparamagnetic particles 102 maybe deposited in multiple layers and on more than one surface of magneticfilm 100.

As is shown in step 604, solvent 610 is evaporated leaving a pluralityof self-assembled superparamagnetic particles 102 on the surface ofmagnetic film 100. Typically, a solvent may be evaporated by applyingheat to the solution, causing the solvent to dissipate as a vapor.However, a solvent may be evaporated at ambient temperatures, and thusthe application of heat is not required for evaporation to occur.According to an embodiment of the invention, the plurality ofsuperparamagnetic particles 102 dispersed within solvent 610 are pouredover magnetic film 100 and magnetic film 100 is heated until all ofsolvent 610 is evaporated. As is shown in step 606, a single layer,i.e., monolayer, of superparamagnetic particles 102 is formed.

Each of the steps of the above deposition technique may be performedmultiple times, together or independently, and in any order, to increasethe number of superparamagnetic particles 102 dispersed on the surfacesof magnetic film 100. Likewise, as was described above in connectionwith the description of FIG. 3, the present deposition techniques may beused to form superparamagnetic layer 306. Similarly, the depositiontechniques may be performed multiple times to for in multiplesuperparamagnetic layers, as was shown in FIG. 4.

In an exemplary embodiment, each of a plurality of superparamagneticlayers are deposited independently, with each additional layer beingdeposited on the previously deposited layer. In this exemplaryconformation, the composition of each superparamagnetic layer may vary,e.g., the composition of the superparamagnetic particles within eachsuperparamagnetic layer may vary.

In addition to the self-assembly technique described above, theplurality of superparamagnetic particles 102 may also be deposited onthe surfaces of magnetic film 100 using conventional sputtering,evaporation, or laser ablation techniques. Conventional sputteringtechniques, as well as co-deposition techniques, may be used to embedsuperparamagnetic particles 102 in non-magnetic host material 302 (FIG.3). Co-deposition techniques, i.e., co-sputtering typically involvessputtering material simultaneously from two or more sputter targets.Thus, according to the teachings herein, superparamagnetic particles 102may be co-sputtered along with the non-magnetic host material comprisingnon-magnetic host material 302, onto magnetic film 100.

In an exemplary embodiment, superparamagnetic particles 102 may beembedded in non-magnetic host material 302 (FIG. 3) by depositinguniform layers, thermally treating the layers to promote interdiffusionof material, and the formation of superparamagnetic particles embeddedin the non-magnetic host material. Superparamagnetic particles 102 willthen be present in non-magnetic surroundings.

When superparamagnetic particles 102 are deposited on the surfaces ofmagnetic film 100, the particles are allowed to self-assemble.Self-assembly of superparamagnetic particles 102 ensures an evendistribution of the particles about the surfaces of magnetic film 100.

As will be described in detail below, the magnetic film of the presentinvention may be employed as at least one, or portions of at least one,of the magnetic layers of a magnetic switching device. By introducingsuperparamagnetic particle 102 to magnetic film 100, an increaseddamping of magnetic film 100, and consequently the magnetic switchingdevice, is realized. The energy from the magnetization precession inmagnetic film 100 is transferred to superparamagnetic particle 102 byexcitation of the magnetization of superparamagnetic particle 102.

For example, when magnetic film 100 is used as a component of a magneticswitching device, increased damping results. As the device is switched,switching energy, namely, fast, large angle magnetization rotations areproduced. Without damping, the magnitude of the switching energy mayresult in the unpredictable, uncontrollable and undesirable switching ofthe element. The superparamagnetic nanoparticles are useful inincreasing the magnetization damping by dissipating the switchingenergy, thus, improving the switching characteristics of the magneticswitching device.

One way of testing the damping properties of magnetic film 100 is tomake a magnetic element test structure suitable for ferromagneticresonance measurements (FMR). FIG. 7 is a diagram illustrating across-section of an exemplary magnetic element test structure suitablefor FMR. For example, the magnetic element test structure may be a thincircular disk. The magnetic element test structure may comprise anoxidized silicon substrate 702, a magnetic film 704 and self-assembledsuperparamagnetic particles 102. The magnetic element test structure mayuse a commercially available oxidized silicon substrate 702 as abuse.Magnetic film 704 may be deposited by direct current (DC)-magnetronsputtering in a high vacuum chamber. Magnetic film 704 may have athickness in the range of about four nanometers to about 60 nanometers.Magnetic film 704 may comprise any suitable magnetic material, includingbut not limited to, Ni₈₁Fe₁₉. Superparamagnetic particles 102 aredeposited on top of the magnetic film 704. As is shown in FIG. 7,superparamagnetic particles 102 are coated with a non-magnetic polymer.

The magnetic damping properties of the magnetic element test structuremay be determined. For a detailed description of the determination ofmagnetic damping properties of a magnetic test structure, seeIngvarsson.

The damping may be expressed in terms of the Gilbert damping parameter,α, that enters the Landau-Lifshitz-Gilbert equation of motion formagnetization:

$\begin{matrix}{\frac{M}{t} = {{{- \gamma}\; M \times H} + {\frac{\alpha}{M}M \times \frac{M}{t}}}} & (1)\end{matrix}$

wherein γ is the gyromagnetic ratio, M is magnetization, H is effectivemagnetic field and t is time. A linearization of equation 1 gives thefollowing form for the susceptibility when the applied DC field isperpendicular to the direction of the alternating current (AC) fieldgenerated by the current loop:

$\begin{matrix}{\chi = \frac{{\gamma^{2}{M\left( {H + {\left( {{4\pi} + \frac{H_{k}}{M}} \right)M}} \right)}} + {\frac{\omega}{\gamma}\alpha}}{\omega_{r,{\alpha = 0}}^{2} - {\omega^{2}\left( {1 + \alpha^{2}} \right)} + {{\omega\alpha\gamma}\left( {{2H} + {\left( {{4\pi} + \frac{2H_{k}}{M}} \right)M}} \right)}}} & (2)\end{matrix}$

wherein H_(k) is an in-plane uniaxial anisotropy field, and surfaceeffects are neglected. The undamped resonance frequency may be definedas:

$\begin{matrix}{\omega_{r,{\alpha = 0}}^{2} = {{\gamma^{2}\left( {H + {\left( {{4\pi} + \frac{H_{k}}{M}} \right)M}} \right)}\left( {H + H_{k}} \right)}} & (3)\end{matrix}$

Exchange effects and radio frequency skin depth effects are negligibleat the relatively low frequency, i.e., less than or equal to threegigahertz (GHz), at which Permalloy™ films resonate and at the geometricdimensions of the test structure of the present invention. Thesaturation magnetization may be obtained using any suitable magnetometrytechnique, such as vibrating sample magnetometry (VSM), alternatinggradient magnetometry (AGM) or super conducting quantum interferencedevice (SQUID)-magnetometry. The anisotropy field can also be determinedby the techniques described above. However, at high frequencies, resultsfor anisotropy can differ substantially from the DC-results. Therefore,the anisotropy field may be obtained from the angular dependence of theresonance frequency. With the parameters defined, equation 2 may be fitto experimental results regarding susceptibility to determine theGilbert damping parameter, α.

In accordance with the teachings of the present invention, magnetic film100 may be patterned and may be employed in a small area structure, andas such, may also exhibit increased coercivity. The coercivity increasesmay be contained by the introduction of superparamagnetic particles 102,as described above. Containing or decreasing the coercivity bydeposition of nanoparticles over such small area structures cantherefore help decrease the switching field and unwanted heating.

The magnetic film of the present invention may be used in a variety ofapplications. For example, the magnetic film may be used in a spin valveor in a magnetic tunnel junction, as well as in magnetic switchingdevices employed in high frequency switching applications. The magneticfilm may be employed, for example, in a magnetic random access memory(MRAM) device, as is described in detail below.

FIG. 8 is a diagram illustrating a simplified cross-section of anexemplary magnetic switching device, magnetic tunnel junction 800. Inthis application, the magnetic switching device comprises magnetic layer802, barrier layer 804 and magnetic layer 806. Magnetic layers 802 and806 are separated by barrier layer 804. Suitable barrier layers comprisenon-magnetic materials, including, but not limited to, oxidizedaluminum. Any suitable technique may be employed to form barrier layer804. In an exemplary embodiment wherein barrier layer 804 comprisesoxidized aluminum, aluminum may first be deposited and subsequentlyoxidized throughout. The resistance of magnetic tunnel junction 800, asmeasured between magnetic layers 802 and 806, is strongly dependent onthe relative magnetization orientation of magnetic layers 802 and 806.In an exemplary embodiment, one of magnetic layers 802 or 806 serves asthe fixed magnetic layer, while the other of magnetic layers 802 or 806serves as the free magnetic layer. The free magnetic layer may be“switched” by reversing the magnetic direction thereof (i.e., a rotationof 180 degrees) by application of magnetic fields or by spin injection.

The magnetic materials of the present invention may be employed aseither, or both of, the fixed magnetic layer or the free magnetic layer.In an exemplary embodiment, a magnetic film comprising superparamagneticparticles on at least one surface thereof is employed as the freemagnetic layer. Further, as was described in conjunction with thedescription of FIG. 3, a spacer layer, i.e., spacer layer 304, may bepresent to spatially separate the superparamagnetic particles from themagnetic film.

The magnetic switching device, as highlighted above in conjunction withthe description of FIG. 8, may be employed in an MRAM device. An MRAMdevice typically comprises a plurality of memory cells and a pluralityof word lines and a plurality of bit lines operatively coupled to thememory cells for selectively accessing the memory cells. At least one ofthe memory cells may comprise the magnetic switching device, as in FIG.8. The magnetic switching device may comprise one or more of themagnetic materials disclosed herein. In an exemplary embodiment, themagnetic switching device includes a magnetic film comprisingsuperparamagnetic particles on at least one surface thereof.

As highlighted above in reference to the description of FIG. 8, one ormore of the magnetic films disclosed herein may be employed as one, orboth, of magnetic layers 802 or 806. The same configurations apply whenthe magnetic switching device is employed in an MRAM device. In anexemplary embodiment, magnetic film 100 with superparamagnetic particles102 on at least one surface thereof is used as the free layer in theMRAM system to achieve favorable switching characteristics, namely,increased damping and decreased coercivity. Each of the magnetic layersof the magnetic switching device employed in the MRAM device have thedimensions of about 100 square nanometers. In an exemplary embodiment,superparamagnetic particles 102 form superparamagnetic layer 306 (FIG.3) on the surface of magnetic film 100 of each magnetic switching deviceemployed in the MRAM device. Thus, in this particular embodiment, giventhe particle size range of the superparamagnetic particles, about 100superparamagnetic particles per superparamagnetic layer 306 aredistributed onto magnetic film 100 in the MRAM structure. Furthermore,magnetic film 100, used in the MRAM structure, may comprise multiplesuperparamagnetic layers, as shown in FIG. 4.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope or spirit of the invention. The following example isprovided to illustrate the scope and spirit of the present invention.Because the example is given for illustrative purposes only, theinvention embodied therein should not be limited thereto.

Example

A magnetic film was synthesized with superparamagnetic layers comprisingsuperparamagnetic particles deposited on the surface thereof. Thedamping properties of the film were tested with FMR, as described above.The superparamagnetic particles used for the test comprised cobalt, andwere formed with an average diameter of about nine nanometers. Themagnetic film was made comprising Permalloy™, Ni₈₁Fe₁₉. The magneticfilm was formed in the shape of a disk with a diameter of about 1.8millimeters and having a thickness of about 30 nanometers. The dampingproperties of the film were tested at three stages of deposition of thesuperparamagnetic particles, namely, prior to deposition; after a firstdeposition and self-assembly of superparamagnetic particles; and after asecond deposition and self-assembly of superparamagnetic particles ofthe second deposition on top of the first. The results are shown plottedin FIG. 9. FIG. 9 is a plot illustrating magnetic susceptibility as afunction of frequency in the magnetic film samples. In FIG. 9,“as-grown” indicates the Permalloy™ film without any superparamagneticparticles present, “1^(st) deposition” indicates the first deposition ofsuperparamagnetic particles deposited on the surface of the Permalloy™film, and “2^(nd) deposition” indicates the second deposition ofsuperparamagnetic particles deposited on top of the firstsuperparamagnetic layer and the Permalloy™ film.

In FIG. 9, increased damping of the magnetic film is indicated by areduced, broadened curve. Thus, the sample having the least amount ofdamping is the Permalloy™ film without superparamagnetic particlespresent. The first superparamagnetic particle deposition on thePermalloy™ film increases the damping of the magnetic film. The seconddeposition of superparamagnetic particles, on top of the first, furtherincreases the damping of the magnetic film. Thus, increasing the amountof superparamagnetic particles on the surface of the magnetic film is aneffective way of increasing the magnetization damping effect on themagnetic film.

FIG. 10 is a plot illustrating the Gilbert damping coefficients as afunction of an applied direct current (static) magnetic field of themagnetic film samples. In FIG. 10, several samples of the Permalloy™film with superparamagnetic particles are shown contrasted with the “asgrown” sample of the Permalloy™ film without superparamagnetic particlespresent. Specifically, the solid triangles (indicated by the symbol “▴”)represent data points from the “as grown” sample of the Permalloy™ filmwithout superparamagnetic particles present. The “as grown” sample isincluded as a reference. The open, inverted triangles (indicated by thesymbol “∇”) represent data points from the “1^(st) deposition” samplehaving a first superparamagnetic layer deposited on the surface of thePermalloy™ film. The open triangles (indicated by the symbol “Δ”)represent data points from the “2^(nd) deposition” sample having asecond superparamagnetic layer deposited on the surface of the firstsuperparamagnetic layer opposite the Permalloy™ film. All other symbols,namely plus symbols (indicated by the symbol “+”), stars (indicated bythe symbol “*”), cross marks (indicated by the symbol “×”) and dots(indicated by the symbol “”), represent data points from samples of thePermalloy™ films with superparamagnetic particles deposited with thesame technique as described above, but with different particle densityand arrangement. In all cases of particles on the surface of themagnetic films, the damping effect increases with increasing magneticfield, i.e., as the particle moments align with the magnetization. Whenthe particle moments align with the magnetization of the film, theparticle moments become more susceptible to the magnetic flux changescaused by the processing magnetization in the film, and thus, couplemore strongly to it. As a comparison, the damping coefficient of the “asgrown” sample is independent of the applied field.

To estimate the effect of superparamagnetic layers on the coercivity ofmagnetic elements, superparamagnetic nanoparticles were deposited on anarray of ellipsoidal magnetic elements. FIG. 11 is an atomic forcemicroscopic image of a section of the ellipsoidal magnetic elementsarray. An array of identical (electron-beam lithographically defined)elements were used to make the magnetic signal large enough to measure.Thus, the average properties of the array were measured. Given the factthat the array was carefully defined, the properties from one element toanother did not vary significantly. Thus, the average properties of thearray provided a good estimate of the properties of the individualelements.

Table 1, below, shows the effects of depositing nine nanometer diameterspherical magnetic nanoparticles, comprising cobalt, on top of arectangular array of Ni₈₀Fe₂₀ ellipsoids (with the dimensions: {shortaxis} 0.24 micrometers by {long axis} 0.84 micrometers by {thickness}five nanometers). As described above, an image of a section of theellipsoidal magnetic elements array is shown in FIG. 11. The ellipsoidsare separated by 1.5 micrometers (edge to edge) in the direction of theshort axis and one micrometer in the direction of the long axis. Inanother case, a non-magnetic polymer layer was deposited under theparticle layer. The non-magnetic polymer layer restricted the particledeposition to one monolayer of particles. In yet another case, a dilutemixture of nanoparticles suspended in octane was deposited. Thattechnique did not limit the deposition to a monolayer, but would limitthe number of layers that may be deposited due to the weak particleconcentration.

TABLE 1 Coercive field, Percentage of Sample: H_(c) (Oersted)“as-deposited array” As-deposited ellipsoid array. 105 100 Array withmonolayer of 75 71 particles. Array with dilute particle 65 62deposition.It is evident from the data in Table 1 that the coercive field of theellipsoid array is significantly reduced by the introduction of theparticle layer on top of it.

1. A method for forming a magnetic film comprising superparamagneticparticles, the method comprising the steps of: forming the magneticfilm; and depositing the superparamagnetic particles on at least onesurface of the magnetic film.
 2. The method of claim 1, wherein thesuperparamagnetic particles are deposited using sputtering techniques.3. The method of claim 1, wherein the superparamagnetic particles aredeposited using evaporative techniques.
 4. The method of claim 1,wherein the superparamagnetic particles are deposited using laserablation techniques.
 5. The method of claim 1, wherein thesuperparamagnetic particles are deposited using self-assemblytechniques.
 6. The method of claim 1, wherein the superparamagneticparticles are deposited along with a non-magnetic host material.
 7. Themethod of claim 1, wherein the depositing step further comprises thesteps of: depositing at least one non-magnetic layer; and forming thesuperparamagnetic particles embedded in the at least one non-magneticlayer by thermal treatment.